This project is devoted to elucidating the mechanisms responsible for the hypercoagulable state and developing methodology for its detection in humans. Activation events are crucial to the definition of a hypercoagulable state. The conversion of zymogens to their enzymatic forms, the activation of co-factors to their potent species, and the acceleration of inhibitors are examples of these critical phenomena. This proposal is therefore directed towards elucidating a particular set of these activation events and utilizing this knowledge to detect a hypercoagulable state in humans prior to the formation of thrombi. Specifically, we intend: a) To determine the biochemical details of the mechanism of antithrombin action and the manner in which heparin accelerates the function of this inhibitor; b) To determine the role of antithrombin in regulating the human coagulation system; c) To determine the role of antithrombin in regulating the fibrinolytic systems; d) To investigate the biochemistry of human Factor V and determine its mechanism of activation; e) To complete the purification and characterization of the prothrombin activation fragments as well as the thrombin precursors. To determine the role of Factor Xa and Factor V in selecting the pathway(s) of prothrombin conversion. f) To develop three separate prototype assays for hypercoagulability based upon our knowledge of the prothrombin activation mechanism and to test their predictive value in detecting a prethrombotic state in humans.